The production of energy from coal and natural gas requires technologies to remove carbon dioxide (CO2), which is a gaseous impurity in both processes. The low cost and global abundance of both coal and natural gas all but ensures the continued use of these two natural resources for energy generation for many years to come. Efforts to develop technologies to improve the removal of unwanted CO2 through the development of selective, high capacity sorbents for CO2 are needed.
To generate energy from coal, integrated gasification combined cycle (IGCC) power plants make use of the water-gas shift reaction. Coal is burned and the carbon monoxide produced is then reacted with water in a reactor containing a catalyst to perform the water-gas shift reaction. This reaction converts water and carbon monoxide to carbon dioxide and hydrogen. The CO2/H2 gas stream produced (called synthetic gas or syngas) typically contains about 35-40 mole percent CO2. An important step in electricity generation at IGCC power plants is the removal of the carbon dioxide generated by the water-gas shift reaction to produce fuel grade or even higher purity hydrogen. The hydrogen is subsequently used to power a combined cycle turbine that produces electricity.
The most widely used method to remove the CO2 from H2 is a pressure swing adsorption cycle with the sorbent being a physical solvent. In a pressure swing adsorption cycle, a CO2/H2 gas stream at high pressure (e.g., 20-45 bar) is passed through the physical solvent resulting in a purified H2 stream exiting the sorbent vessel. The adsorption portion of the cycle is stopped prior to breakthrough of a targeted level of CO2. A desorption step is then performed to regenerate the physical solvent.
Physical solvents separate CO2 from other gases based on a difference in solubility. Because there are only weak interactions between the CO2 and the physical solvent, the CO2 can be easily removed from the physical solvent by reducing the pressure. While there are several different physical solvents in use today, polyethylene glycol dimethyl ether (available under the trade designation SELEXOL) is the most commonly used. While the adsorption selectivity for CO2 is high, the solubility of CO2 in SELEXOL at 20 bar and 25° C. is only about 9.6 weight percent. Although the solubility amount can vary depending on the temperature and pressure used in the process, the ability to capture a higher percentage of CO2 per mass of sorbent while maintaining selectivity over other gases such as hydrogen would be highly advantageous.
Natural gas production requires an extensive set of processes to purify the natural gas to a useable fuel. Typical impurities include acid gases (such as hydrogen sulfide and sulfur dioxide), water, and carbon dioxide. Carbon dioxide is typically present in natural gas at a level close to 5 volume percent. While the most common method to remove CO2 from methane is a pressure swing adsorption cycle, the low partial pressure of the CO2 in the mixture makes the removal of CO2 with physical solvents impractical. A stronger interaction between the CO2 and solvent is required. As such, chemical solvents are typically used. The most widely used chemical solvent is an aqueous solution of ethanol amine. In a single pressure swing adsorption cycle, ethanol amine can separate/capture about 5 percent of its mass in CO2. While the strong interaction of the CO2 with the chemical solvent allows for the efficient removal of the CO2 from the gas stream, regeneration of the chemical solvent requires heating. This heating step tends to render the overall process energetically expensive.
Polymeric materials prepared from divinylbenzene and maleic anhydride have been known for many years. Many of these polymeric materials are prepared by a process called macroreticulation, which refers to a process of making polymeric beads using suspension polymerization. These processes involve forming droplets of an organic phase suspended in an aqueous phase. The suspended organic phase includes the monomers and an optional porogen. The maleic anhydride content in the final copolymer has been low, however, because this monomer tends to undergo hydrolysis and leave the organic phase. Attempts to reduce the hydrolysis reaction have included replacing the aqueous phase with glycerol or other polar solvents. Macroporous copolymers have been prepared.